Fig. 1. Geometric relationship of light reflection

Fig. 2. Microfacet reflection and imaging principle under coaxial light

Fig. 3. Measurement scheme of BRDF on cutting surface

Fig. 4. Measurement equipment of BRDF. (a) Physical map; (b) turning samples; (c) plain milling samples

Fig. 5. Measuring data of surface morphology (color) and images (grayscale) of samples. (a)(d) Ra=3.2 μm, turning surface; (b)(e) Ra=1.6 μm, turning surface; (c)(f) Ra=0.8 μm, turning surface; (g)(j) Ra=3.2 μm, plain milling surface; (h)(k) Ra=1.6 μm, plain milling surface; (i)(l) Ra=0.8 μm, plain milling surface

Fig. 6. Relationship between BRDF value and normal declination angle θ under coaxial light for surfaces with different roughnesses. (a) Ra=3.2 μm, turning surface; (b) Ra=1.6 μm, turning surface; (c) Ra=0.8 μm, turning surface; (d) Ra=3.2 μm, plain milling surface; (e) Ra=1.6 μm, plain milling surface; (f) Ra=0.8 μm, plain milling surface

Fig. 7. Comparison between measurement data and model calculation data of BRDF on cutting surface. (a) Ra=3.2 μm, turning surface; (b) Ra=1.6 μm, turning surface; (c) Ra=0.8 μm, turning surface; (d) Ra=3.2 μm, plain milling surface; (e) Ra=1.6 μm, plain milling surface; (f) Ra=0.8 μm, plain milling surface